1. Technical Field
The present disclosure relates generally to the logging of subsurface formations surrounding a wellbore using a downhole logging tool, and particularly to using the logs to estimate the anisotropic resistivity.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
Certain existing resistivity logging tools have at least one transverse antenna. That is, the magnetic dipole moment of the transverse antenna is perpendicular to the longitudinal axis of the tool. For example, one model of the Schlumberger Technology Corporation's PERISCOPE™ logging tools measures the propagation of electromagnetic signals with an array of transmitter and receiver coils that includes two tilted receivers and one transverse transmitter. The tool obtains directional information, and also detects anisotropic resistivity.